Bone is known to be a dynamic organ which constantly cycles between formation and resorption for reconstruction in order to change its own morphology and to maintain blood calcium levels. Healthy bone maintains an equilibrium between bone formation by osteoblasts, bone resorption by osteoclasts, and its bone mass constant. In contrast, when the equilibrium between bone formation and bone resorption is lost, a metabolic bone disorder such as osteoporosis can develop (Endocrinological Review 13, 66-80, 1992; and Principles of Bone Biology pp. 87-102, 1996, Academic Press, New York).
Many factors involved in regulation of bone metabolism have been reported, including systemic hormones and local cytokines, and they serve together to form and maintain bone (Endocrinological Review 13, 66-80, 1992; and Endocrinological Review 17, 308-332, 1996). A change in bone tissue with aging is widely recognized as a cause of osteoporosis, but the mechanism of its development encompasses various factors, for example, a lower secretion of sex hormones and an abnormality of receptors for the hormones, expression of aging genes, failure to differentiate into osteoclasts and/or osteoblasts and dysfunction of those cells, and thus, as a physiological event due to aging, it is poorly understood. Osteoporosis is largely divided between osteoporosis after menopause due to a lower secretion of estrogen and senile osteoporosis due to aging, but advancement of basic research on the mechanisms of regulation of bone formation and bone resorption is essential to elucidate the mechanism of its development and to develop therapeutic agents.
Osteoclasts are multinucleate cells derived from hematopoietic stem cells, they release chloride and hydrogen ions on the bone surface to which they adhere to acidify the space between the bone surface and the cells themselves (American Journal of Physiology 260, C1315-C1324, 1991). This causes decomposition of calcium phosphate and activation of acid proteases, leading to bone resorption.
Osteoclast precursor cells have been found to be differentiated into osteoclasts by stimulation with RANKL (receptor activator of NF-κB ligand) expressed on the cell membrane of osteoblasts/stromal cells present on the surface of bone (Proceedings of the National Academy of Science of the United States of America 95, 3597-3602, 1998; and Cell 93, 165-176, 1998). It has been shown that RANKL is a membrane-bound factor produced by osteoblasts/stromal cells, its expression is regulated by a bone resorption factor; RANKL induces differentiation of preosteoclastic cells into multinucleate osteoclasts (Proceedings of the National Academy of Science of the United States of America 95, 3597-3602, 1998; and Journal of Bone and Mineral Research 23, S222, 1998). Furthermore, knockout mice devoid of RANKL have been found to develop a typical osteopetrosis, which has verified that RANKL is a physiological inducer for differentiation into osteoclasts (Nature 397, 315-323, 1999).
To treat a bone disorder or shorten the duration of treatment, bisphosphonates, activated vitamin D3, calcitonin and its derivatives, hormone preparations such as estradiol, SERMs (selective estrogen receptor modulators), ipriflavone, vitamin K2 (menatetrenone), calcium preparations and the like are currently used. However, these drugs have not always exhibited a satisfactory therapeutic effect, and thus there has been a desire to develop more potent drugs.
Dendritic cells (referred to as “DC” hereinafter) are specialized antigen-presenting cells of the immune system and distributed throughout the entire body. The dendritic cell-specific transmembrane protein (referred to as “DC-STAMP” hereinafter) is a protein extending across the cell membrane of dendritic cells, that has been cloned from the cDNA library of monocyte-derived DCs (European Journal of Immunology 30, 3585-3590, 2000). One human DC-STAMP cDNA has been reported (GenBank Accession No: NM—030788) and two murine DC-STAMP cDNAs have been reported which are a long sequence cDNA containing the third exon (GenBank Accession No: AB109560) and a splice variant having a short third exon (GenBank Accession No: AB109561), respectively. An amino acid sequence homology of about 74% has been identified between the human DC-STAMP and the murine DC-STAMP. As a result of hydrophobicity analysis for the amino acid sequences, the DC-STAMPs are predicted to have seven transmembrane domains. The murine splice variant having a short third exon is considered to have the seventh transmembrane domain deleted, and is thus denoted by DC-STAMP ΔT7 hereinafter.
DC-STAMP is reported to be more highly expressed following inactivation of mononuclear phagocytes with IL-4, but less highly expressed following their inactivation with dexamethasone (Immunogenetics 53, 105-113, 2001). However, the association of DC-STAMP with differentiation into osteoclasts still remains to be elucidated.